1. Field of the Invention
The invention relates to a dry film type of a halogen-free thermo-curing photosensitive resin composition, which can be used as a photoresist in printed circuit boards. The resin composition of the invention is particularly suitable for use in the surface of printed circuit boards and semiconductor packages as a protective dry film solder mask.
2. Description of the Prior Art
In recent years, due to the development trend of electronic products towards smart, portable, high function and high density and the demands of electronic constructions for high I/O count, miniaturization, and small area, the technology regarding printed circuit boards must be continually improved, and the physical properties of related materials also need to be improved.
After the circuits of printed circuit boards are finished, they should be coated and protected with an insulating resin film, so as to avoid oxidization and short circuits during welding. The protective coating film for the surfaces of printed circuit boards is referred to as solder mask. Due to the more strict conditions for processing and applications, in addition to the physical properties regarding insulation, heat resistance of solders, high rigidity, and chemical resistance, a solder mask should also exhibit the properties of high resolution, fast development, electroplating resistance, electroless plating resistance, and high temperature and moisture resistance. In order to achieve the above high-functional physical properties, many solutions for improving solder masks have been proposed in the prior art. For example, Japanese Laid-Open Patent Appln. No. 11-21327 discloses a light curing resin obtained via the reaction of epoxy resin/(meth)acrylic acid and a monocarboxylic acid compound/anhydride, wherein the monocarboxylic acid compound has more than two light curable unsaturated groups, such that the light curability of the composition could be improved, thereby reducing the exposure amount. However, for a high-efficiency exposure machine, this does not significantly save the exposure time nor does it significantly enhance the bonding and developing speed. In another aspect, Japanese Laid-Open Patent Appln. No. 10-274849 discloses a solder photoresist ink composition, but its development speed is slow, and the physical properties regarding electroless plating resistance and high temperature and moisture resistance are poorer. Moreover, Japanese Laid-Open Patent Appln. No. 5-202332 discloses a composition containing a photosensitive resin (with more than two acrylate groups and more than one carboxylic acid group in one molecule), which has a high resolution, and excellent electrical properties, but the molecular weight of the photosensitive resin is less than 1000, and the touch dry property is poor.
Furthermore, previous wet solder masks are produced by means of direct heat baking after screen printing to cure the film. However, since the solder mask may penetrate onto the copper surface at the circuit joint during printing and curing, resulting in problems associated with welding parts and relevant applications, the solder mask is normally used in simple circuit boards, and dry film solder masks are used instead in manufacture.
In addition, since all plastic products burn easily, flame-retardants will be added. In the past, since the flame-retardants containing a halogen achieved a good balance between cost and performance and could be used in a broad spectrum, such flame-retardants played a dominant role in the application to plastics. However, with the increasing concern about environmental protection, the use of the flame-retardants containing a halogen becomes limited due to the irritant and corrosive gases of dioxin or benzofuran produced during burning the flame-retardants, as well as the carcinogenic effect of these gases. At present, the flame-retardants are developing towards multifunctional purposes. People have paid more attention to improving the flame retardant efficiency, reducing the amount, and eliminating the risk to health and to the environment. Thus, “halogen-free” has become a main tendency in the development and application of flame-retardants.